Yes it is sin. The reason is that this person was born a male, not a female. His problem with his idenity stems from a spiritual issue not a creation one. He needs to seek help in the spiritual area and turn from any thoughs that might cause him to be lead into further sin.
...If a person decides taking their own life is the answer then they simply show that deep spiritual deadness...
...except usually those spiritual issues stem from the prejudice of Christians that refuse to accept that gender dysphoria is a medical condition, and that a person can have a genuine mismatch between the individuals anatomical gender and their emotional, spiritual, and mental sense of their gender. Since a persons innate sense of gender is unchangeable, the only possibility for bringing the gender of their mind and body into conformity is to change the body. Withholding treatment often leads to deep depression which could trigger suicidal thoughts.
Our Lord is compassionate to know our frailties and to forgive -- unless we turn from his forgiveness to stick our noses in our fellow Christians' lives and judge them without compassion or mercy -- whereupon His wrath is unleashed upon us, as He shows in the Parable of the Unforgiving Servant...
Many religious people cling to prejudice and hatred against individuals afflicted with Gender Dysphoria. These attitudes reinforce societal acceptance of harassment of transgender individuals, and is in turn partly responsible for the high rate of murder committed against transgender people. Christians should live as examples of Christ-like love in order to draw more people to salvation.
Often Christians say ultra spiritual slogans like "If you pray enough God will heal you", "Changing ones gender is going against God", or "they're just mutilating their body".
Statements like these show ignorance of what Gender Dysphoria and Gender Reassignment Surgery is, as well as prejudice and personal insecurities. You cant pray away gender dysphoria any more than other birth defects or abnormalities. You cant point to a specific verse that eliminates all doubt that having a sex change is sin. GRS is a medical procedure to repair and heal, precisely opposite to the meaning of the word mutilate.
There also seems to be the widespread opinion that people just go get a sex change the same way you get fast food. I can assure you that standards of care have been established and tested over several decades. These require months of councilling before beginning any treatment. An individual must also live full time in the opposite gender role for at least a year before surgery is allowed. It's also expensive, so it takes more time to save money for the treatment. Thus the individual has ample time to realize if gender reassignment surgery is right or wrong for them.
We see many obvious birth defects in the world around us. Why must so many Christians single out one particular medical condition as being an affront to God if we seek treatment?
If there are medical conditions such as gender dysphoria then the church should support the person involved.
I know there are indeed medical conditions such as gender dysphoria. From personal experience I also know that there are cases where gender reassignment genuinely heals the incongruity of mind and body. This is a well documented medical condition, and is a matter for the individual and his/her doctor(s), not the church.